Small-signal heterodyne mixers with excessive injection amplitudes

The theory of small-signal heterodyne mixers operating with very large injection potentials is described. The mixer properties investigated are the conversion transconductance, the relative amplitudes of the difference-frequency and injection frequency components in the output, and the amplitude of the injection modulation-frequency component in the output in the case where the injection potential is amplitude-modulated to a small degree, as by power-supply ripple or noise, for example. The theoretical analysis is based upon 2- and 3-segment piecewise-linear approximations of the current and incremental transfer conductance curves, so that the mixer can be treated as a switch-type or commutator-type modulator. The method of constructing the approximations from the original curves is described. The result is a set of universal performance curves for mixers of the particular class chosen to illustrate the method. The method can be extended to other classes of mixers.